Nations not meeting international guidelines are shown in parentheses.

Nations in which more than ten percent of the population was excluded from testing are shown with a *. Latvia is designated LSS because only Latvian-speaking schools were tested, which represents less than 65 percent of the population.

Nations in which a participation rate of 75 percent of the schools and students combined was achieved only after replacements for refusals were substituted, are shown with a O .

The international average is the average of the national averages of the 26 nations.